The present invention relates generally to a joint assembly for connecting together sections of heat insulated high temperature fluid carrying conduits and more particularly to an improvement in an assembly of this type.
Where conduit service for carrying high temperature fluids, for example extremely hot water or steam, is required, it is often desirable and sometimes necessary to use a heat insulated conduit for this service. A quite satisfactory conduit of this type is disclosed in U.S. Pat. No. 3,563,572 issued to D. W. French on Feb. 16, 1971 and assigned to Johns-Manville Corporation, assignee of the present application.
The fluid carrying insulated conduit disclosed in the French patent is essentially composed of concentric lengths of pipe consisting of an inner fluid carrying pipe or core, an outer protective pipe or casing spaced concentrically around the inner pipe core and a layer of thermal insulating material positioned concentrically therebetween. Two conduits sections of this type are connected together by means of a composite coupling arrangement including an innermost tubular sealing sleeve or coupling which is positioned concentrically around adjacent end sections of the inner fluid carrying pipes or cores of adjacent conduit sections. In this regard, the cores project out beyond their associated concentrically disposed insulation layers and outer casings. Fluid seals are provided between the innermost coupling and the projecting core sections by means of a pair of stainless steel spring loaded gasket rings of trifluorinated ethylene (E. I. DuPont's "TEFLON" or Allied Chemical's "HALON").
As set forth in the French disclosure, these trifluorinated ethylene sealing rings may require heat deformation to effectively seat and seal them for complete sealing between the innermost coupling and projecting core sections. As further stated, this may be automatically provided when the sealing rings are exposed to the temperatures of the hot fluid carried in a conduit. However, this patent does recognize that minor leaks are possible until the sealing rings have been adequately exposed to the elevated temperatures of the carried fluid. Thus, to prevent possible initial leakage, the French patent suggests the inclusion of a pair of secondary sealing rings. In accordance with the French patent, since these secondary seals may only be required until the unit has been exposed to high temperatures for seating the primary seals, i.e., the trifluorinated ethylene seals, the secondary seals may be composed of a composition designed only to endure relatively low temperatures such as ethylene propylene rubber which will perform continuously at temperatures up to or slightly above about 250.degree. F. When continuously exposed to these temperatures, the secondary seals will eventually become permanently damaged. However, the trifluorinated ethylene compound will have had sufficient exposure to the heat to effect proper seating.
It has been found not only to be desirable but important to the overall performance of the arrangement disclosed in the French patent to prevent the occurrence of this initial leakage just discussed since it is possible for the moisture leaking past the seals to enter into the insulation layers comprising part of the connected conduits. Moisture reaching the insulation layers could adversely affect the performance of the overall conduit arrangement. Accordingly, as stated, the French patent discloses the utilization of secondary sealing rings. To prevent the leakage in the event the secondary sealing rings are not used, it has been found necessary to either preheat the trifluorinated ethylene rings, i.e., the primary seals, before subjecting the conduits to high temperature fluid under pressure, for example the hot water or steam, or by gradually rising the temperature of the fluid within the conduits, under low pressure conditions, so as to sufficiently heat up the primary seals before the fluid is raised to the higher operational pressures. This preheating procedure is strongly advisable in subsequent startups even if the secondary seals are used. More specifically, the secondary seals may be initially used but probably will become damaged and ineffective as a result of being subjected to the high temperatures within the conduits. Accordingly, when the conduits are subsequentially cooled (through lack of service) and thereafter are required to carry the hot fluid a second time, i.e., during a second start-up, it is strongly advisable that the aforediscussed preheating procedure be followed to prevent the aforediscussed initial leakage.
As will be seen hereinafter, the present invention is directed to an improvement in the overall arrangement disclosed in the French patent. This improvement substantially minimizes or eliminates the possibility of moisture reaching the aforediscussed layer of insulation material comprising part of the heat insulated conduit in the event there is the initial leakage discussed above and, in fact, makes it possible to eliminate the secondary seals and/or preheating procedure otherwise used to prevent the initial leakage.